The present invention relates to a screen having matrix access, which makes it possible to represent an image by splitting the same into a set of dots or elements whose appearance varies as a function of control signals applied to electrodes delimiting these elements. This screen makes use of a material of which the optical properties may be modulated electrically. The control signals are fed to the electrodes through non-linear resistors acting as switching elements.
The principle of the flat screen consists in dividing the screen into M identical elements which are generally square or rectangular. These elements may be addressed individually. The definition of the screen is a function of the number of dots able to receive an information. Each dot must consequently be exposed to an electrical field. This may easily be envisaged for a screen comprising some tens of dots. Direct access utilising a wire to each element becomes practically impossible for high definition screens (exceeding 1.5.times.10.sup.4 dots). An easily realisable display of the matrix type has consequently been contemplated. Each screen element is then defined by the intersection of two grids of orthogonal conductors referred to as lines and columns: the number of connections changes from M.times.N to M+N. The saving on connections is considerable for a high resolution screen.
The addressing of a screen element by means of control voltages applied to the line and column in question need not be retained if a temporal multiplexing technique is adopted which renders it possible to refresh the screen state by recurrence. This technique is based on a persistence effect which may be physiological or available within the screen element. In the case of liquid crystal display devices, the elementary cell may be likened to a capacitor of which the time constant is adequate to maintain the charge between two successive transient addressing actions. In order to apply the control voltage in a short time, a non-linear resistor, meaning an element of the varistor type which is practically an insulator below a voltage threshold and becomes increasingly conductive beyond this threshold, is connected in series with the capacitive cell. A convenient manner of producing the varistor elements collectively consists in utilising as a substrate, a block of varistor material occupying the same area as the screen. This procedure results in producing a parasitic capacitance able to impair the satisfactory operation of the screen, across the terminals of each varistor element.
With a view to overcoming this disadvantage, the invention makes provision for machining the varistor material substrate in such a manner as to leave proud no more than islets or small regions forming the non-linear resistors. To restore the plane form of the substrate, the depressions surrounding the islets are filled with a dielectric material having a substantially lower permittivity than that of the varistor material.